Die steel alloy



United htates Patent DIE STEEL ALLOY Ernest C. Kron, Toledo, Ohio, assignor to National Lead lZompany, New York, N. Y., a corporation of New ersey No Drawing. Application March 18, 1955 derial No. 495,338

2 Claims. c1. 75-126 This invention relates to a new die steel and more specifically to a steel alloy having properties particularly desirable for use in the die casting industry.

The instant die steel alloy will have a composition falling within the limits shown in column A of the following table, and a preferred alloy composition is that shown in column B.

Die steel alloys within the composition indicated and particularly that alloy having the preferred composition possess deep hardening properties when air quenched so that heavy and light sections will have very near the same hardness. The instant steel alloy hardens to martensite and bainite from a quenching temperature of 1850 F. or higher with only a trace of carbide in the structure. For example, when employing 0.20% carbon the desired hardness for die steel (about 444 Brinell) will be obtained on air quenching, and tempering at 850 P. will not change the hardness.

The uniformity of hardness and the other desirable properties of the alloy of this invention are obtained by balancing the carbon content in relation to the other alloying elements. Since the alloy is employed as a die steel in its fully hardened condition, it is important that the carbon content be rigidly controlled. The amount of carbon determines the ultimate hardness, and it has been found that the use of 0.15% to 0.25% carbon will produce the hardness which has proved most desirable for die casting die steels.

The range of alloy composition may be within that given in the table; however, it has been found that for best results both the chromium and molybdenum, should neither be on the low side or the high side of the range at the same time. The chromium and molybdenum are added to develop the desired air hardening properties and the high temperature characteristics required in a die steel. The silicon and manganese content within the indicated ranges is also in accordance with good tool and die steel practice.

It is important to limit the vanadium content of the instant alloy to a maximum of 0.35%. If the vanadium content is increased above this amount, the vanadium combines with the carbon to form vanadium carbide. When part of the carbon is combined with the vanadium in this manner, there obviously is less carbon available ice for the martensite and bainite with the result that a lower hardness is obtained than is desired when the alloy is heat treated at 1850 F. 'The vanadium carbide contained in the heat treated structure of an alloy containing over 0.35% vanadium will be dissolved at higher welding temperatures and the result is that a zone will be formed around the weld which zone is harder than the normally heat treated steel alloy some distance away from the weld. The alloy of the instant invention having the vanadium content so controlled can be heat treated at 1850 F. with only a trace of residual carbide. Die steels previously employed, however, have generally possessed a microstructure containing a considerable amount of excess undissolved carbide.

There are a number of comparative properties of the instant die steel alloy which make it superior to other alloys commonly used to manufacture die casting dies. Among these is the fact that the die steel of this invention can be welded without a zone forming around the weld which is harder than the originally hardened steel. Die steels presently in use will form a very hard zone around the weld which in many cases will approach a hardness of 600 Brinell. To eliminate such a hard zone from the dies prepared from these materials, it is often necessary to apply a number of reheating treatments to the die and such treatments are costly and time consuming. Likewise, previous die steels when air quenched from 1850 F. often possess a hardness varying from about 461 Brinell to 534 Brinell and it is then necessary to subject the die steel to a very exacting tempering operation in order to obtain the desired final hardness of 444 Brinell. Very frequently it requires two or three tempering operations before the desired hardness is obtained. The difliculty of obtaining uniform hardness in a large die is quite obvious. However, when the die steel of this invention is employed, such difficulties are not encountered. As previously indicated the instant alloy will harden on air quenching from 1850 F. to the desired hardness of 444 Brinell, and even subsequent tempering at 850 F. to relieve strains does not change the hardness of the die steel.

The new die steel also has higher resistance to notch impact than have the die steels generally heretofore employed in the industry. The increased resistance results in a tougher die which has less tendency to crack at sharp corners as are often found in the die itself.

The new die steel is easy to polish since its microstructure contains practically no undissolved carbide or hard compounds which in previous die steels have been known to break out on polishing, thus causing pits to form on the polished surface.

As many variations are possible within the scope of this invention it is not intended to be limited except as defined by the following claims.

I claim:

1. A die steel alloy consisting essentially of 0.15 to 0.25% carbon, 0.25 to 1.25% silicon, 0.20 to 1.00%

manganese, 3.50 to 7.00% chromium, 1.00 to 3.00% molybdenum, 0.15 to 0.35% vanadium, up to 0.03% sulfur, up to 0.03% phosphorus, and the balance iron, said alloy characterized in that it will harden on air quenching from at least 0 F. to the hardness required in die casting applications, said alloy being resistant to notch impact, easily polishable and further characterized in that it can be welded without a zone forming around the weld which is harder than the originally hardened steel.

2. A die steel alloy consisting essentially of 0.20% carbon, 0.65% silicon, 0.50% manganese, 5.25% chromium, 2.00% molybdenum, 0.25% vanadium, up to 0.03% sulfur, up to 0.03% phosphorus, and the balance iron, said alloy characterized in that it will harden on air quenching from at least 1850 F. to a hardness of 444 Brinnel, said alloy being resistant to notch impact, easily polishable, and further characterized in that it can be welded without a zone forming around the weld which is harder than the originally hardened steel.

References Cited in the file of this patent FOREIGN PATENTS Switzerland June 3, 1941 OTHER REFERENCES Transactions of the American Society of Metals (1), vol. 27, 1939, page 1092. Pub. by the American Society for Metals, Cleveland, Ohio.

Transactions of the American Society for Metals (2), vol. 34, 1945, pages 593-595.- Pub. by the American Society for Metals, Cleveland, Ohio. 

1. A DIE STEEL ALLOY CONSISTING ESSENTIALLY OF 0.15 TO 0.25% CARBON, 0.25 TO 1.25% SILICON, 0.20 TO 1.00% MANGANESE, 3.50 TO 7.00% CHROMIUM, 1.00 TO 3.00% MOLYBDENUM, 0.15 TO 0.35% VANADIUM, UP TO 0.03% SULFUR, UP TO 0.03% PHOSPHORUS, AND THE BALANCE IRON, SAID ALLOY CHARACTERIZED IN THAT IT WILL HARDEN ON AIR QUENCHING FROM AT LEAST 1850*F. TO THE HARDNESS REQUIRED IN DIE CASTING APPLICATIONS, SAID ALLOY BEING RESISTANT TO NOTCH IMPACT, EASILY POLISHABLE AND FURTHER CHARACTERIZED IN THAT IT CAN BE WELDED WITHOUT A ZONE FORMING AROUND THE WELD WHICH IS HARDER THAN THE ORIGINALLY HARDENED STEEL. 